Control system for synchronous motors



June 9, 1942. v s. c. EWING 2,285,917

CONTROL SYSTEM FOR sYNcmwNoUs MOTORS Filed June 20, 1941 Figl.

Inventor: Samuel C. Ewing.

by fiis Attorney.

Patented June 9, 1942 CONTROL SYSTEM FOR SYNCHBONOUS MOTORS Samuel 0. Ewing, Sootia, N. 1., amino! General Electric Company, a corporation of New York . Application June 20, 1941, Serial No. 398,890

-8Claims.

My invention relates to control systems for synchronous motors and particularly to control systems for automatically removing the field excitation from a synchronous motor when it is pulled out of step and for subsequently re-e'stablishing the field excitation to resynchronize the motor.

One object of my invention is to provide an improved arrangement for preventing the windings of a synchronous motor from being damaged due to the failure of the motor to resynchronize within a predetermined time after being pulled out of step.

In accordance with my invention, I provide an arrangement of apparatus for limiting the number of times that the automatic control apparatus can attempt to resynchronize the motor after it has been pulled out of step.

' My invention will be better understood from the following description when taken in connection with the accompanying drawing, Fig. 1 of which diagrammatically illustrates a synchronous motor control system embodying my invention, and Fig. 2 01' which diagrammatically illustrates a modification of a portion of the system'shown in Fig. 1, and the scope of my invention will be pointed out in the appended claims.

Referring to Fig. l of the drawing, I represents a synchronous motor having an armsture winding 2 and a field winding 3. In order to simplify the disclosure, I have shown my invention in connection with a full voltage starting system whereby the synchronous motor is started as an induction motor by connecting the motor armature winding directly to an alternating current supply circuit while the field winding of the motor is short-circuited through a suitable resistor. It is to be understood, however, that my invention is not limited to any particular arrangement for starting the motor. As shown in the drawing, the armature-winding 2 is arranged to be connected directly to the alternating current supply circuit 1 by means of a suitable switch 5 having a closing coil 0 which is arranged to be connected across a suitable control circuit when a control switch I is closed. As shown in the drawing, the control switch I is a manually operated switch, but it will be obvious to those skilled in the art that it may be automatically controlled in any suitable manner so that it is closed whenever it is desired to start the motor. In the circuit of the closing coil 6, I also provide the normally closed contacts of a switch 8, which may be controlledin any suitable manner, so

that it is opened when it-is desired to stop the motor from a remote control point. The switch I, when closed, completes through its auxiliary contacts I a locking circuit for the closing coil 0 so that the starting switch I may be opened without efiecting the opening of the switch I. The field winding 3 of the motor I is arranged to be connected to a suitable source of excitation ill by means of a field switch I I when it is closed. when the field switch II is open, its auxiliary contacts, I! connect a suitable discharge resistor ll across the terminals of the field winding 3. The field switch II is provided with an operating coil II which, when energized, moves the switch from its open position to its closed position. The energization of the closing coil is v effected by means of a time relay ll whenever the switch I is closed and the field switch II is open. This result is accomplished by having the energizing circuit of the time relay I! include the auxiliary contacts is of the field switch ii and the contacts ll of the switch 5. when the field switch ii is in its closed position, its auxiliary contacts I! control a shunt circuit around the contacts 20 of the time relay i! so that the closing coil i4 is not deenergized due to the deenergization of the time relay i! which is efiected by the field switch ll moving to its closed position.

' In series with the contacts 26 of the relay II which are in the shunt circuit around the contacts of the time relay ii, I also provide the contacts I! of a suitable out-of-step responsive relay Il shown as a power-factor relay, the windings of which are connected to the armature cir cult of the motor I in any suitable manner, examples of which arewell known inthe art, so that, as long as the power-factor of the motor is above a predetermined lagging value, the relay II maintains its contacts I! closed. a

In order to limit the number of times the field switch I I can be reclosed when the motor is pulled out of step, I also provide in the energizing circuit of the closing coil 8 the contacts 22 of a time relay II which is arranged to be energized each time the field switch I I is opened while the switch I is closed. This time relay 2i is designed in any suitable manner so that it will open its contacts I2 when-the relay is energized for a predetermined time interval a predetermined number of times with less than a predetermined time interval between any two successive energizations thereof. In the particular arrangement shown in Fig. 1, an energizing circuit for the time relay 2| is completed by the opening of the field switch cuit for the closing coil 6 of the switch 5.

II, if at the same time a relay 24 is energized, The relay is arranged to be energized in response to the first closure of the field switch II, and when energized, completes a locking circuit for itself which remains completed as long as the closing coil 6 of the switch I is energized.

The operation of the embodiment of my invention shown in Fig. 1 is as follows:

when it is desired to start the motor I, the control switch is closed to complete through the contacts of the control switch 8 and the contacts 22 of the time relay 2| an energizing circlosing of the switch .5 connects the armature winding 2 of the motor I directly across the alternating current supply circuit 4 so that the motor starts and accelerates as an induction motor; The closing of the auxiliary contacts 9 on the switch 5 completes a shunt circuit around the contacts of the control switch I so that this control switch may be opened without effecting the deenergization of the closing coil 6.

During the starting operation of the motor I, a circuit is completed for the motor field winding 3 through the discharge resistor I3 and the auxiliary contacts I2 of the field switch Theclosing of the auxiliary contacts II on the switch 5 completes through the auxiliary contacts I6 of the field switch M an energizing circuit for the time relay I5 which, after being energized for a predetermined length of time that is long enough to allow the motor to accelerate to approximately its synchronous speed, closes its contacts 26 and completes an energizing circuit for the closing 'coil I4 of the field switch 7 through the contacts I! of the switch 5. The closing of the field switch connects the field winding 3 to the source of excitation I so that the motor I pulls into synchronism. The clos' ing of the auxiliary contacts I8 of the field switch I! completes through the contacts i9 01 the power-factor relay 2!! a shunt circuit around the contacts 26 of the time relay I so that the subsequent opening of the contacts 26 does not deenergize the closing coil I4 oi the field switch H. The contact 26 of the relay I5 opens in a predetermined time interval after the coil of relay i5 is deenergized by the contact IB' of the field switch II. This is to allow the motor to reach a steady state cdndition after the field switch II is initially closed. The closing of the auxiliary. contacts 21 of the field switch II completes an energizing circuit for the control relay 24 through the auxiliary contacts II of the switch 5. By closing its contacts 20, the relay 24 completes a shunt circuit around the contacts 2'! of the field switch so that the subsequent open ing of the fieldswitch I I while the switch 5 is closed does not deenergize the control relay 24.

As long as the motor I remains in synchronism, the power-factor of the motor armature circuit is such that the power-factor relay 20 maintains its contacts I9 closed. When, however, the motor is pulled out of synchronism, the power-factor oi the motor armature circuit becomes sufilclently lagging to cause the power-factor relay 20 to open its contacts I9 and thereby interrupt the energizing circuit of the closing coll I4 0! the field switch II. The field switch II immediately moves to its-open position, thereby disconnecting the field winding 2 from the source of excitation III and reconnecting the discharge resistor I3 across the terminals of the field winding 3. By closing its contacts I6, the field switch II again completes an energizing circuit for the The time relay I5, and by closing its contacts ll, completes an energizing circuit for the time relay 2| through the contacts 2| oi the control relay 24 and the contacts II of the switch-II. Both of the time relays I! and 2| start their timing operations, but since the operating time of the time relay 2| is much longer than the operating time of the time relay II, the latter will close its contacts 26 and effect, in' the manner heretofore described, the closing of the field switch II before the timing relay 2| can eiiect the opening of contacts 22. The reclosing of the field switch I again causes the motor to pull into step, and if it remains in synchronism after-the first reclosure of the field switch II, the circuit of the time relay 2| will remain open at the contacts 20 for a sufilcient length of time to allow the time relay 2| to be restored to itsnormal position. However, if the motor is stilloverloaded so that it is again pulled out of step as soon as the field switch II is reclosed, the power-factor relay 20 again opens its contacts I9 and effects the opening of the field switch II in the manner heretofore described so that the time relays I5 and 2| are reenergized. After the field switch I I is opened for a predetermined time, the time relay I5 again effects the closing thereof. In this manner, the field switch II is intermittently reclosed in an attempt to resynchronize the motor until either the motor remains in synchronism or the time relay 2| is energized for a sufilcient length of time to open its contacts 22. If the motor remains in synchronism after any reclosure of the field switch I], the time relay 22 is deenergized for a sumcient length of time to allow it to be restored to its normal position. If, however, a sufi'icient number of openings of the field switch occur to cause the relay '2I to open its contacts 22, the circuit of the closing coil 6 of the switch 5 is opened so that the motor armature winding 2 is disconnected from the supply circuit 4, thereby causing the motor to be shut down. Instead of using 'a time relay 2| of the type diagrammatically shown in Fig. 1, a notching relay 2| of the type While I have, in accordance with the patentstatutes, shown and described my invention as applied to a particular system and as embodying various devices diagrammatically indicated, changes and modifications will be obvious to those skilled inthe art, and I therefore aim in the appended claims to cover all such changes and modifications'as fall within the true spirit and scope oi my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, an alternating current circuit, a source or direct current, a synchronous motor having its armature winding connected to said alternating current circuit and its field winding connected to said source, means for disconnecting said field winding from said source when said motor falls out of step and subsequently reconnecting said field winding to said source, and means responsive to a predetermined number of disconnections of said field winding from said source for effecting the disconnection of said armature winding from said circuit.

2. In combination, an alternating current circuit, a source of direct current, a synchronous motor having its armature winding connected to said alternating current circuit and its field winding connected to said source, means for disconnecting said field winding from said source when said motor falls out of step and subsequently reconnecting said field winding to said source while said armature winding remains connected to said circuit, and means responsive to a predetermined number of disconnections of said field winding from said source for effecting the disconnection of said armature winding from said circuit.

3. In combination, an alternating current circuit, a source of direct current, a synchronous motor having its armature winding connected to said alternating current circuit and its field winding connected to said source, means for disconnecting said field winding from said source when said motor falls out of step and subsequently reconnecting said field Winding to said source while said armature winding remains conneoted to said circuit, and means responsive to a predetermined number of disconnections of said field winding from said source within a pre determined time interval for. effecting the disconnection of said armature winding from said circuit.

4. In combination, an alternating current circuit, a source of direct current, a synchronous motor having its armature winding connected to said alternating current circuit and its field winding connected to said source, means for disconnecting said field winding from said source when said motor falls out of step and subsequently reconnecting said field winding to said source, a control device, means for actuating said device in response to each disconnection of said field winding from said source, and means responsive to a predetermined number of actuations of said device for disconnecting said armature winding from said circuit.

5. In combination, an alternating current circuit, a source of direct current, a synchronous motor having its armature winding connected to said alternating current circuit and its field winding connected to said source, means for disconnecting said field winding from said source when said motor falls out of step and subsequently reconnecting said field winding to said source, a control device, means for actuating said device in response to each disconnection of said field winding from said source, and means responsive to a predetermined number of actuations of said device within a predetermined time interval for disconnecting said armature winding from said circuit.

6. In combination, an alternating current circuit, a synchronous motor having its armature windingconnected to said circuit, means for decreasing the field excitation of said motor when it falls out of step and for subsequently increasing the field excitation to resynchronize the motor, and means responsive termined number of changes in the excitation of said motor for disconnecting said armature winding from said circuit.

'1. In combination, an alternating current circuit, a synchronous motor having its armature winding connected to said circuit, means for decreasing the field excitation of said motor when it falls out of step and for subsequently increasing the field excitation to resynchronize the motor, and means responsive to a predetermined number of changes in the excitation of said motor within a predetermined time interval for disconnecting said armature winding from said circuit.

8. In combination, an alternating current cir-. cuit, a synchronous motor having an armature winding and a field winding, a source of excitation, a field switch for connecting said source to said field winding, means for connecting said armature winding to said circuit and for subsequently closing said field switch, a relay responsive to the closing of said field switch, means for opening and subsequently reclosing said field switch when said motor falls out of step, and means controlled by said relay and a predetermined number of operations of said'field switch for disconnecting said armature winding from said circuit.

SAMUEL C. EWING. v

to a prede- 

